To my knowledge, no country in the world outright bans their citizens from investing in ICOs. ICO's rather not have to deal with the regulatory bullshit from the U.S. and other countries regarding their citizens: mainly taxes.
Whenever you don't know an answer to a question regarding why someone else did something, the answer is almost always directly or indirectly because of $$$$ or love.
But you can look at each of the ICO, all of which limits the participation of American citizens, and it's not allowed to participate in ICO.